Electron Affinity vs Electronegativity: Key Differences & Table
Electron Affinity is essential in chemistry and helps students understand various practical and theoretical applications related to atom and ion formation. Knowing about electron affinity allows students to predict which elements will gain electrons easily and form negative ions, an important step in understanding periodic table trends and reactivity.
What is Electron Affinity in Chemistry?
Electron affinity refers to the energy change that occurs when an atom in the gaseous state gains an extra electron to form a negative ion. This important concept appears in chapters related to atomic structure, periodic table trends, and chemical bonding, making it a foundational part of your chemistry syllabus. A higher (more negative) electron affinity means the atom attracts electrons more strongly.
Molecular Formula and Composition
Unlike molecules with fixed formulas, electron affinity is a property rather than a specific compound or molecule. It applies to individual atoms, especially nonmetals like chlorine (Cl) or oxygen (O), and is measured in kilojoules per mole (kJ/mol). The general process can be written as: X(g) + e⁻ → X⁻(g).
Preparation and Synthesis Methods
You cannot “prepare” electron affinity itself, but you can calculate or measure it in laboratories using advanced instruments. Methods like spectroscopic techniques, the Born-Haber cycle, and thermochemical data are used to determine the electron affinity of different elements.
Physical Properties of Electron Affinity
Electron affinity is a numerical value usually expressed in kJ/mol. For most elements, electron affinity values are negative, meaning energy is released when an electron is added. For example, chlorine’s electron affinity is about -349 kJ/mol, which is the highest among all elements. Noble gases have values close to zero or even positive, meaning they do not attract extra electrons easily.
Chemical Properties and Reactions
Although electron affinity is not a chemical itself, it predicts how easily an element forms negative ions through reactions. For instance, elements with high electron affinity form stable negative ions (anions) like Cl⁻ or O²⁻, which then participate in ionic bonds with metals. Second electron affinity, such as when O⁻(g) gains another electron to form O²⁻(g), requires energy (positive electron affinity) due to electron repulsion.
Frequent Related Errors
- Confusing electron affinity with electronegativity or electron gain enthalpy.
- Assuming all nonmetals have the same electron affinity values.
- Ignoring periodic trends and group exceptions (like fluorine and noble gases).
- Thinking the process always releases energy (second electron affinity is endothermic).
Uses of Electron Affinity in Real Life
Electron affinity helps predict reactivity and bonding in industrial processes, such as manufacturing bleach, semiconductors, and fertilizers. It also guides chemists in designing chemical reactions for electronics, water treatment, and pharmaceuticals. Elements with high electron affinity are used to make strong oxidizing agents.
Relevance in Competitive Exams
Students preparing for NEET, JEE, and Olympiads should be familiar with electron affinity, as it often features in concept and calculation questions regarding the periodic table, atomic trends, and chemical bonding. Knowing electron affinity trends is also useful for CBSE board exams, especially when comparing it to electronegativity or ionization energy.
Relation with Other Chemistry Concepts
Electron affinity is closely related to topics such as electronegativity, electron gain enthalpy, and periodic table trends. Understanding it helps students build links between atomic structure, the nature of chemical bonds, and element reactivity.
Step-by-Step Reaction Example
1. Write the electron affinity addition process for chlorine:Cl(g) + e⁻ → Cl⁻(g) ΔE = -349 kJ/mol
2. State what happens:
The chlorine atom in the gaseous state attracts an extra electron, releasing energy and forming a stable Cl⁻ ion.
3. Explain second electron affinity using oxygen:
O(g) + e⁻ → O⁻(g) ΔE₁ = -141 kJ/mol (energy released)
O⁻(g) + e⁻ → O²⁻(g) ΔE₂ = +780 kJ/mol (energy absorbed)
4. Final Answer: First electron affinity is exothermic; second is endothermic due to repulsion.
Lab or Experimental Tips
Remember electron affinity by linking it to “energy released when an electron is gained.” Vedantu educators often advise drawing quick tables of values or visualizing group/period trends with arrows in live classes for fast revision. Use “up and right on the periodic table means higher electron affinity, except for noble gases.”
Try This Yourself
- Compare the electron affinity of fluorine and chlorine—who has the higher value and why?
- List three elements with very low electron affinity values.
- Is the electron affinity of noble gases positive, zero, or negative? Explain why.
- Draw a simple periodic table trend arrow map for electron affinity.
Final Wrap-Up
We explored electron affinity—its meaning, process, and why it matters for reactions and the periodic table. You now understand how electron affinity connects to ion formation, exceptions across the periodic table, and related chemistry topics. For more exam-focused notes, solved problems, and live guidance on this and other chemistry concepts, check out Vedantu’s other topic pages and join a live class!
FAQs on Electron Affinity Explained: Trends, Definition & Examples
1. What is electron affinity in chemistry?
Electron affinity is the energy change that occurs when a neutral atom or molecule in the gaseous state gains an electron to form a negatively charged ion. A more negative electron affinity value indicates a stronger attraction for electrons.
2. What trend does electron affinity follow in the periodic table?
Electron affinity generally increases across a period (from left to right) and decreases down a group (from top to bottom) in the periodic table. However, there are exceptions to this trend, particularly among the halogens.
3. Which element has the highest electron affinity?
Chlorine (Cl) generally exhibits the highest electron affinity among all the elements. Although fluorine is more electronegative, its smaller size leads to increased electron-electron repulsion, resulting in a lower electron affinity than chlorine.
4. How is electron affinity different from electronegativity?
Electron affinity measures the energy change associated with gaining an electron by a single atom in the gaseous state. Electronegativity, on the other hand, describes an atom's ability to attract electrons within a chemical bond. While related, they are distinct concepts.
5. Why is the electron affinity of fluorine less than that of chlorine?
Despite being more electronegative, fluorine's smaller atomic size leads to significant electron-electron repulsion in its compact valence shell. This repulsion counteracts the strong nuclear attraction, resulting in a lower electron affinity compared to chlorine.
6. What factors affect electron affinity?
Several factors influence an atom's electron affinity, including:
• Nuclear charge: A higher nuclear charge leads to stronger attraction for electrons.
• Atomic size: Larger atoms have weaker attraction for electrons due to increased distance from the nucleus.
• Electronic configuration: Atoms with stable electron configurations (like noble gases) have low electron affinities.
• Shielding effect: Inner electrons shield outer electrons from the nucleus, reducing the effective nuclear charge.
7. What is the significance of electron affinity in chemical bonding?
Electron affinity plays a crucial role in determining the stability and type of chemical bonds formed. It influences the formation of ionic bonds (through electron transfer) and contributes to the properties of covalent bonds (through electron sharing) by influencing the relative electronegativities of atoms involved.
8. What are some examples of elements with high electron affinity?
Elements with high electron affinities are typically found in Group 17 (halogens), including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). These elements readily accept electrons to achieve a stable octet configuration.
9. What is meant by second electron affinity, and why is it usually positive?
The second electron affinity refers to the energy change when a second electron is added to a negatively charged ion (anion). It is usually positive because adding an electron to an already negatively charged ion requires overcoming electrostatic repulsion, making it an endothermic process.
10. How can electron affinity help predict the reactivity of elements?
Elements with high electron affinities tend to be highly reactive as they readily accept electrons to form stable ions or compounds. This is particularly evident in the high reactivity of the halogens, which readily form ionic bonds with metals.
11. Why are electron affinity values sometimes reported as positive?
A positive electron affinity value indicates that energy is required to add an electron to an atom, rather than energy being released. This is often the case for elements with already stable electron configurations, where adding an electron would destabilize the atom.
12. How does electron affinity relate to the formation of ionic compounds?
The difference in electron affinity between elements is a key factor in the formation of ionic compounds. Elements with significantly different electron affinities (e.g., a metal with low electron affinity and a nonmetal with high electron affinity) are likely to form ionic bonds through the transfer of electrons.
